Typical gate valves are defined by a valve body that is formed to define a valve chamber with a flow passage being defined in the valve body in intersecting relation with the valve chamber. To control the flow of fluid through the flow passages, a gate member is provided having a flow port defined in one portion thereof and a solid blocking portion defined at another portion thereof. The gate member typically reciprocates within the valve chamber between an open position where the flow port is in registry with the flow passage through the valve body and a closed position where the solid blocking portion of the gate blocks the flow of fluid through the flow passage. In order to maintain a sealed relationship between the gate and valve body typically upstream and downstream seat rings are retained within seat pockets defined in the valve body about the flow passage. These seat rings establish sealing engagement with planar sealing surfaces defined at the upstream and downstream portions of the gate.
It is well known in the valve art that movement of a circular gate port relative to circular seat rings develops openings of small dimension between the gate port and seat rings during the initial portion of opening movement and during the terminal portion of closing movement. Under circumstances where these openings are of small dimension, such as during opening and closing, and especially when the valve is in high pressure service and the fluid controlled thereby incorporates a quantity of abrasive medium such as sand, abrasive slurry, line scale, etc., the downstream seat of the valve is likely to become worn to the extent that the sealing capability thereof will fail. Erosion of the downstream seat is due to concentrated impingement of high velocity fluid flowing through the small opening between the upstream seat ring and the gate port. This impinging, high velocity flow of the fluid medium is directed in concentrated relation against a small area portion of the downstream seat. Moreover, each time the valve is cycled, i.e. opened or closed, this high velocity fluid impingement occurs and, within a relatively short period of time, the downstream seat will become eroded and the sealing capability of the valve will deteriorate to the extent that valve repair is required. In many cases, gate valves are manufactured with extremely hard and durable upstream and downstream seat rings in order that erosive failure be retarded as much as possible.
In many cases, the stringent requirements of the service condition to which the valve is to be subjected, dictates that gate valves meet certain rather severe minimum certified standards in order to qualify the valve design for use in the particular intended service. For example, a particular test procedure identified as API-14-D requires that all manufacturers of API-14-D Class 2 valves (wellhead surface safety valves for offshore service) certify one valve at an independent test laboratory. The basic objective of the testing requirements is to qualify the valve design for use in sandy service. The test procedure for certifying the test valve is as follows: